1. Field of the Invention
This invention relates generally to wire bonding lead frames to semiconductor dice and, more specifically, to wire bonding a lead frame to a semiconductor die using a laser beam to provide the energy necessary to bond or fuse a wire to a lead of a lead frame and to a contact pad of a semiconductor die. The method and apparatus of lead-to-chip bonding herein described is applicable to either a conventional lead frame and chip arrangement or a lead-over-chip (LOC) arrangement, in any instance, where the lead of a lead frame is directly or indirectly bonded to the contact pad of a semiconductor chip.
2. State of the Art
Various types of semiconductor chips are connected to lead frames and subsequently encapsulated in plastic for use in a wide variety of applications. A conventional lead frame is typically formed from a single continuous sheet of metal, typically by metal stamping operations. The conventional lead frame includes an outer supporting frame, a central semiconductor chip supporting pad and a plurality of leads, each lead having, in turn, a terminal bonding portion near the central chip supporting pad. Ultimately, the outer supporting frame of the lead frame is removed after the wire bonds between the contact pads of the semiconductor chip and the leads are made and the semiconductor chip and lead frame have been encapsulated.
In an LOC lead frame the lead frame has no central chip supporting pad with the semiconductor chip being held in position with respect to the lead frame and leads by means of adhesive strips secured to the leads of the lead frame and the semiconductor chip.
A typical apparatus and method for forming the wire bonds between the contact pads on a semiconductor chip and the leads of lead frames is illustrated in U.S. Pat. No. 4,600,138. As disclosed, a bond head is shown moving from a first bonding location to a second bonding location. The end of the wire is bonded to the first bonding location by the bond head. The bond head moves vertically away from the first bonding location to draw a length of wire necessary to make the wire bond. The bond head is then moved to the second bonding location with subsequent bonding of the wire to the second bonding location. The bond head is then used to pull and subsequently break away the remaining wire from the second bonding location. The bond head is then ready to be moved to another first bonding location for effecting another wire bond.
Typically, the bond head is heated to assist the formation of the wire bond. The heat and subsequent pressure applied by the bond head fuses the end of the wire to the contact pad. Ultrasonic vibration in conjunction with a heated bond head may also be used to affect a wire bond. Typically, there is a single bond head for making all of the wire bonds of the semiconductor chip. As should be recognized by those skilled in the art, such an operation is inherently mechanical in nature and thus limited to the speed of movement of the mechanical device.
One method of speeding up a conventional wire bonding process is to provide the heat necessary to effect a wire bond by utilizing heat generated from a laser beam to heat the bond head. Such apparatuses are disclosed in U.S. Pat. No. 4,534,811 to Ainslie et al., and U.S. Pat. No. 4,845,354 to Gupta et al. However, as the number of connections per semiconductor chip increase and the size of the leads decrease, such a bonding tool becomes impractical.
It has also been recognized in the art to use laser beams to form a lead-to-chip bond. For example, a method for reflowing solder to bond an electrical lead to a solder pad using a laser, in which the solder pad, rather than the terminal, is irradiated by the laser beam, is disclosed in U.S. Pat. No. 4,926,022 to Freedman. In addition, in U.S. Pat. No. 5,274,210 to Freedman et al., electrical connections may be made by coating conductive elements with a non-flux, non-metallic coating material making it possible to use a laser for bonding. The laser is either moved in a continuous sweep around all of the connections or pulsed.
It has also been recognized in the art to use a laser beam to bond the bumps of an integrated circuit to a tape automated bonding (TAB) tape lead. TAB, in general, has been one attempt in the art to increase the speed and efficiency of the chip-to-lead bonding process. For example, in U.S. Pat. No. 4,978,835 to Luijtjes et al. and U.S. Pat. No. 5,049,718 and U.S. Pat. No. 5,083,007 to Spletter et al., a laser beam is directed onto the ends of the leads of a TAB tape.
None of the previously mentioned prior art references, however, have successfully utilized laser light to reduce the mechanical limitations of the bonding process. More specifically, prior art devices either move the device relative to the laser for every bond, or a single laser beam to every bonding site. Thus, it would be advantageous to provide an apparatus and method for forming wire bonds using a laser in which the laser need not move for each bond and where more than one bond can be made substantially simultaneously.